The present invention relates to an advanced treatment process of service water and secondary treatment waste water, and to a treatment of industrial water or waste water. More particularly, it relates to a membrane filtration treatment process of water using ozone that constantly and effectively provides a filtrate with a certain high quality.
A typical conventional water purification process is a coagulation sedimentation process. This process comprises first adding chlorine or sodium hypochlorite to collected raw water, oxidizing iron or manganese so as to make them insoluble and at the same time preventing microorganisms from increasing and from causing adverse effects in the purification process, then adding a coagulant appropriate to coagulate suspended substances, flocculating the coagulated substances for sedimentation separation, removing overflowed flocs by sand filtration, and cleansing the raw water. When the raw water contains few suspended substances, it may be treated according to a process comprising line-mixing coagulants followed by a sand filtration and a process comprising floating suspended substances by pressure.
However, it is impossible to achieve the water purification level required in the recent semiconductor industry and the protozoa removal level required in waterworks only by the conventional coagulation sedimentation processes. As the quality of raw water to be treated in waterworks has been debased year by year, there has been a demand, for example, to remove colored elements and elements having a foul odor and taste, to remove harmful organic substances such as agricultural chemicals and environmental hormones, to remove protozoas such as Cryptosporidium and Giardia, and to reduce chlorine disinfection by-products such as trihalomethanes. For satisfying such demands, advanced treatment such as biological treatment, treatment with ozone and treatment with activated carbon has been initiated.
Recently, membrane filtration processes using an ultrafiltration (UF) membrane and a microfiltration (MF) membrane have been adopted in smaller waterworks because of their advantages over the coagulation sedimentation process, such as a high ability to remove impurities, germs and protozoas sufficiently, high reliability and capability of automatic operation. Further, combinations of the membrane filtration treatment with the above advanced treatment are being studied.
However, the membrane filtration treatment requires regular cleansing of chemicals since continuous filtration causes clogging of the membrane so that a filtration flux gradually decreases. In order to reduce the frequency of cleansing of chemicals to as low as possible, it is necessary to carry out troublesome pretreatment such as coagulation sedimentation or to set the membrane filtration flux small. Therefore, its applicable field is limited by economic concerns. Further, according to the membrane filtration treatment, the protozoas contained in the raw water such as Cryptosporidium are completely removed so that the filtrate becomes safe. However, the condensed waste water produced by the membrane filtration contains concentrated protozoas, and therefore rigid oversight is required for the disposal of the waste water.
In order to avoid the above drawbacks, for instance, U.S. Pat. No. 5,271,830 and PCT International Patent Publication No. WO97/10893 disclose processes for preventing a membrane from clogging by feeding ozone upstream of the membrane and simultaneously improving water quality. However, for achieving sufficient effects in such a treatment, the ozone must be supplied excessively in anticipation of the change in the quality of the raw water. Therefore, these processes are economically disadvantageous. Further, the increase in the feeding amount of ozone unpreferably results in the formation of by-products and there is an increased load on the activated carbon in the following step, i.e., the use of activated carbon is increased by reacting with the ozone remaining in the filtrate.
On the other hand, a smaller feeding amount of ozone decreases the membrane filtration flux due to the clogging of the membrane. As a result, the water quality is not sufficiently improved. Moreover, if the raw water quality is changed, the membrane filtration flux is greatly fluctuated and the quality of the treated water is also fluctuated. Accordingly, it is difficult to ensure a certain amount of treated water with a certain quality all the time.
An object of the present invention is to provide a process of membrane filtration treatment of water using ozone, wherein a filtrate having a certain high quality is obtained while a large filtration flux is maintained regardless of the fluctuation in the quality of raw water.
Another object of the present invention is to provide a process of membrane filtration treatment of water with ozone, wherein an amount of ozone remaining in the filtrate is kept small so as to conduct the following water treatment steps effectively.
A further object of the present invention is to provide a water treatment process, wherein treated water with a high quality is obtained using a compact system and harmful wastes are not produced.
The water treatment process of the present invention comprises adding ozone into raw water and filtering the raw water using an ozone-resistant membrane, wherein the concentration of ozone present in the water filtered through the ozone-resistant membrane is detected and an amount of ozone to be fed to the raw water is continuously and automatically controlled so that the concentration of ozone is adjusted to a prescribed value.
Embodiments of the water treatment process of the present invention as described above include:
a water treatment process comprising detecting a concentration of ozone present in the water filtered through an ozone-resistant membrane and controlling continuously and automatically an amount of the ozone to be fed to the raw water so that the concentration of ozone is adjusted to a prescribed value in the range of 0.05 to 1.0 mg/l;
a water treatment process, wherein the water filtered through the ozone-resistant membrane is further treated with activated carbon;
a water treatment process, wherein the water filtered through the ozone-resistant membrane is further treated using a reverse osmosis membrane;
a water treatment process, wherein, before the treatment with a reverse osmosis membrane, the filtrate is subjected to aeration or treatment with activated carbon, or sodium thiosulfate is added to the filtrate;
a water treatment process, wherein a coagulant is added to raw water prior to the filtration with an ozone-resistant membrane; and
a water treatment process, wherein a pH value of the raw water is controlled to enhance the effect of the coagulant.